Puberty, probably if this is the age where their bodies naturally mature. I'm sure as elite athletes they're all training at home and probably have a regimen developed by their trainers. So if they do have growth spurts now, it will be because it's their time naturally, not due to a lack of training.
As for losing their ultra-c, it's probably possible because they don't have ice training, but also equally possible that they'd get them back relatively quickly. Alena got her 3A in what - 3 weeks (apart from the first attempt)? Sasha learned her 4T in a few weeks as well AFAIK in the time between JGPF and JW? And Anna took almost an entire year off training not just on ice but off (she was in much worse shape than now) and regained all her triple and 4Lz in a season.
Many skaters get injured and have to take a good chunk off in the absence of a pandemic but regain their jumps relatively quickly. For most it's just triples, but for someone like Sasha who has never fallen on a triple in international competition, I'd expect quads, at least the 4T, would be as easy as other's relearning their triples.
